I have a hobbyist website in English with high quality content (yes, real hobbyist website) no ads (AdSense, etc.) I got an offer from a marketing agency to put backlinks (or rather a short paragraph with two links) on the page to online casino / gambling / poker. I don't care about search engine traffic, so I don't mind any penalization. What would be the starting price for such link per month (including invoice)? I have experience with selling and buying normal links, but online gamling is a rather "shady" area so I have no idea what to ask. Thanks for any suggestions.
Find out who the client is before agreeing. Some offer silly prices for large multinational firms where you would be wiser to charge more. The actual cost of the link is entirely up to you and can depend on how long the link should remain, page rank of your site and how high on the rankings your website is placed.